Incorporating lithium salicylate or the like into a grease

ABSTRACT

AN ALKALI METAL SALT AND PARTICULARLY A LITHIUM SALT OF SALICYLIC ACID OR RELATED HYDROXY BENZOIC ACID IS INCORPORATED INTO A LUBRICATING OIL OR GREASE AS AN ANTIOXIDANT BY SPONIFYING AN ALIPHATIC ALCOHOL ESTER OF THE HYDROXY BENZOIC ACID AND THEN REMOVING THE WATER AND ALCOHOL FORMED IN THE REACTION. IMPROVEMENTS OVER PRIOR ART METHODS INCLUDE BETTER PARTICLE SIZE DISTRIBUTION AND LOWER COST.

United States Patent O 3,711,407 INCORPORATING LITHIUM SALICYLATE OR THE LIKE INTO A GREASE Richard J. Plumstead, Mississauga, Ontario, Canada, as-

signor to Esso Research and Engineering Company, Linden, N .J. No Drawing. Filed Nov. 18, 197i), Ser. No. 90,878

Int. Cl. C10m 5/14 US. Cl. 25241 6 Claims ABSTRACT OF THE DISCLOSURE An alkali metal salt and particularly a lithium salt of salicylic acid or related hydroxy benzoic acid is incorporated into a lubricating oil or grease as an antioxidant by saponifying an aliphatic alcohol ester of the hydroxy benzoic acid and then removing the water and alcohol formed in the reaction. Improvements over prior art methods include better particle size distribution and lower cost.

BACKGROUND OF THE INVENTION This invention concerns an improved method for incorporating an alkali metal salt of a hydroxy benzoic acid such as salicylic acid into a lubricant composition to impart antioxidant properties thereto. It is taught in US. Pat. 2,951,808 to employ alkali metal salts of hydroxy benzoic acids as antioxidants for grease compositions. The hydroxy benzoic acids, the metal salts of which are of concern in the present invention, have the general formula:

(IOOH R OH wherein R is hydrogen or short chain alkyl of insufficient length to render the salt soluble in oil, e.g. methyl.

The hydroxy group can be in the ortho, meta, or para position relative to the carboxyl group, although the ortho position is preferred, e.g. as in salicylic acid. The sodium and lithium salts of salicylic acid are particularly useful.

Since the salts discussed above are insoluble in mineral oil, it is necessary to use some method of dispersing such salts into lubricants in the form of small particles. The prior art methods that have been used for incorporating such salts into greases include (a) co-neutralizing salicylic acid with the soap-making fatty acid during grease manufacture, (b) adding an aqueous solution of the salicylate salt to the grease and subsequently dehydrating the mixture, and (0) adding to a finished grease a previously formed salicylate salt that has been ground to a fine powder of the desired particle size. Experience has shown that only the last-named method gives particles small enough so as not to interfere with lubrication. However, the required micro-grinding of the salt renders this method undesirably expensive.

DESCRIPTION OF THE INVENTION In accordance with the present invention, it has now been found that lithium salicylate and related alkali metal salts of hydroxy benzoic acids can be formed as extremely small particles evenly dispersed throughout a lubricating oil or grease by saponifying an aliphatic alcohol ester of the hydroxy benzoic acid and then removing the water and alcohol that are formed in the reaction. The reaction can be conducted in a mineral oil composition that contains a dispersant. When the salicylate salt is to be incorporated into a grease, the reaction can be conducted either ice" in the finished grease or during the neutralization step in the soap-making stage of grease manufacture.

The aliphatic alcohol whose ester is used in this invention will preferably be one whose boiling point is not so high as to prevent its being driven olf during the reaction, unless it is desired to have all or a portion of the alcohol remain in the composition. 'Reaction temperatures can range from about F. to about 550 'F., more usually from about 180 F. to about 380 F. In the case of greases, residual alcohol in the grease may be objectionable in that it may exert excessive softening characteristics. On the other hand, some softening action may actually be desirable where it is necessary to have a high thickener content and cost is a secondary consideration.

Generally the esters will be of C to about C aliphatic alcohols, but preferably they will be about C to C aliphatic alcohols, including methanol, ethanol, propanol, isopropanol, secondary butanol, Z-methyl butanol, 2,2-dimethyl propanol, etc. Particularly preferred are the esters of C to C aliphatic alcohols.

The hydroxy benzoates of this invention will be incorporated in the lubricant in an amount ranging from about 0.01 to about 10 wt. percent, or more usually from about 0.1 to about 5 wt. percent based on the total weight of the composition.

The additives of the invention can be used in any of the conventionally employed mineral lubricating oils as well as in greases prepared using any of such base oils.

Lubricating greases generally comprise a major proportion of a lubricating oil base into which has been incorporated from about 2 to 30 weight percent, based on the total weight of the composition, of a salt, soap, or a mixed-salt or soap-salt complex thickener, or a polymeric thickener such as polyethylene, or inorganic thickeners such as graphite, colloidal asbestos, carbon black, clays, etc. Such salt and soap thickeners are generally metal salts of monocarboxylic acids, such as fatty acids, although sulfonic acids are also frequently used. The soapsalt and mixed-salt thickeners are generally complex thickeners which are prepared by the co-neutralization of a high molecular weight fatty acid, and/or an intermediate molecular Weight fatty acid, and a low molecular weight fatty acid, with metal bases, generally alkali metal or alkaline earth metal bases.

The high molecular weight fatty acids or aliphatic monocarboxylic acids useful for forming the soaps, soapsalt complexes and mixed-salt complexes, include naturally occurring or synthetic, substituted and unsubstituted, saturated and unsaturated, mixed or unmixed fatty acids having about 12 to 30, more usually 16 to 22, carbon atoms per molecule. Examples of such acids include stearic, hydroxy stearic, such as 12-hydroxy stearic, dihydroxy stearic, polyhydroxy stearic, and other saturated hydroxy fatty acids, arachidic, oleic, ricinoleic, hydrogenated fish oil acids, and tallow acids.

Intermediate molecular weight fatty acids include aliphatic, saturated or unsaturated, unsubstituted, monocarboxylic acids that contain 7 to 12 carbon atoms per molecule, e.g., capric, caprylic and lauric acids.

Suitable low molecular weight acids include saturated and unsaturated, substituted and unsubstituted aliphatic monocarboxylic acids having about 2 to 6 carbon atoms. These acids include fatty acids such as acetic, propionic, and similar acids, including their hydroxy derivatives such as lactic acid.

The metal component of the soaps, salts or soap-salt complex thickeners can be any soap-forming metal such as aluminum, but generally is an alkali metal such as lithium, potassium, or sodium; or an alkaline earth metal such as calcium, strontium, barium, or magnesium.

The nature of this invention and the manner in which it can be practiced will be better understood when reference is made to the following examples, which include a preferred embodiment.

EXAMPLE 1 One preferred method for incorporating dilithium salicylate into a grease by the process of this invention is asfollows.

About one-third of the oil required in the finished grease is charged to a hot-oil-heated kettle. About wt. percent of IZ-hydroxy stearic acid based on the finished composition is then charged to the kettle. This mixture is heated to about 180 F. to dissolve the acid in the oil. At this point, about 1.5% by wt. of lithium hydroxide monohydrate dissolved in water is added. This results in a thickening of the mixture due to the formation of the lithium soap. The contents are stirred for about /2 hour. After this time 3.3% of methyl salicylate is added and, after this is stirred in, another 1.7% of lithium hydroxide dissolved in water is added. The contents are then heated to about 300 F. during which time the mixture is dehydrated and the methyl salicylate reacts With the lithium hydroxide to form dilithium salicylate. After the addition of the methyl salicylate it may be desirable to add the second portion of lithium hydroxide in stages so that very little free water is in the mixer at any one time. Without this precaution the dilithium salicylate formed may dissolve in the free water and crystallize out on the kettle walls during dehydration. At 300 F. the balance of the oil is slowly added while the temperature is caused to rise to about 385 F. At this temperature the heat is turned 011?, that is, the flow of hot oil is discontinued and cooling is begun. The kettle contents are stirred while they cool down to about 160 F. and any additives such as rust inhibitors and supplementary oxidation inhibitors may then be incorporated.

EXAMPLE 2 The dillithium salt of salicylic acid was added to five separate grease compositions by various methods at a concentration of 3% by weight. Greases A, B, C and D were prepared from about 10 wt. percent of 12-hydroxy stearic acid, 1.5 of lithium hydroxide monohydrate, 3 wt. percent dilthium salicylate, and 85.5% lubricating oil. The method of preparation for these greases was similar to that described in Example 1, with slight modifications. Grease Ewas prepared by the method of Example 1.

In preparing Grease A, the salicylic acid was charged to the kettle, along with the hydroxy stearic acid. Both of the acids were then neutralized with lithium hydroxide. About 3.2 parts of lithium hydroxide were used for ten parts of fatty acid and three parts of salicylic acid.

To prepare Grease B the oil, the hydroxy stearic acid, and the lithiumhydroxide were heated together at about 180 F. The resulting soap concentrate was then dehydrated in the kettle by heating to about 275 F. In a separate vessel, three parts of salicylic acid was neutralized with an aqueous solution of 1.7 parts of lithium hydroxide monohydrate. The neutralized mixture was added slowly to the soap concentrate in the grease kettle at 275-300 F. At this temperature the water was flashed off, leaving the dilithium salicylate dispersed in the soap. Then the balance of the oil was added, the temperature was raised to 385 F., and the contents of the kettle were then cooled. (Alternatively, the soap concentrate could be heated up to 385 F. and the mixture of dilithium salicylate in water could be added to the grease at about 300 F. on the cooling portion of the cycle.)

In preparing Grease C and Grease D the powdered dilithium salicylate was added to a dilithium hydroxy stearate grease during the cooling cycle at about 200 F.

The finished greases were then inspected for their size distribution of dilithium salicylate particles, using Federal Test Method 3005.3. The results obtained are shown in Table I which follows.

TABLE I.-EFFECT OF INCORPORATION METHOD ON PARTICLE SIZE Particle size distribution, number of particles per cc. of grease, microns It is to be noted from the data in Table I that the particle size was undesirably large in those cases where the salt was formed by co-neutralizing salicylic acid in situ with the fatty acid during grease manufacture, where the salt was dispersed by adding a water solution of the salt to the grease and then dehydrating, and where the salt was added to the finished grease as a powder that had been ground in a ball mill. Jet milling involves impinging particles against a metal wall at high impact speeds, and gives a finer product than when the powder is ball milled. It will be seen that the best results were obtained when using the process of the present invention, because all of the particles in this grease were less than 30 microns in size.

EXAMPLE 3 This example describes the use of the present invention for the preparation of a grease having a ratio of 1 mole of hydroxystearic acid to 0.75 mole of azelaic acid and 0.67 mole of methyl salicylate. Into 300 grams of VI. mineral lubricating oil having a viscosity at F. of 550 SUS there were dispersed 92 grams of 12-hydroxystearic acid and 43.2 grams of azelaic acid at a temperature of 200 F. Then 31.7 grams of lithium hydroxide hydrate (LiOH-H O) was added and the reaction was continued at ZOO-240 F., after which the reaction product was dehydrated by heating it to 300 F. The free alkali content of the dehydrated soap concentrate was 0.18%, which indicated that neutralization of all of the carboxyl groups was essentially complete. After the soap concentrate had been dehydrated, 30.4 grams of methyl salicylate and 17 grams of lithium hydroxide monohydrate were added at 220 F. and the reaction was continued for 1 hour at this temperature. In this reaction methyl alcohol was evolved and dilithium salicylate was formed in the concentrate. After dehydration had been conducted at 300 F. the grease was cooled and oil was added to obtain the desired consistency. The grease is identified in the accompanying Table II as Grease F.

A second grease was prepared using the same proportions of reactants as for the preparation of Grease F, except that the methyl salicylate and lithium hydroxide were reacted in a finished base grease rather than in the soap concentrate. To prepare the finished base grease for later incorporation of dilithium salicylate, the dehydrated soap concentrate prepared as described above was heated to 390-400 F. for 30 minutes to improve the dispersion and then cooled while additional oil Was added to give the desired consistency. Then 30.4 grams of methyl salicylate and 17 grams of lithium hydroxide monohydrate were reacted with the base grease for one hour at 220 F. followed by dehydration at 300 F. giving a product hereinafter identified as Grease G.

COMPARATIVE EXAMPLE A grease having the same components as Grease G was prepared by dispersing in the base grease (described in Example 3) 32.7 parts by weight of dilithium salicylate that had been preformed and then ground to a fine powder by jet milling. This grease is identified in Table II as Grease H.

The compositions of each of the greases F, G and H are given in Table II, which follows, along with certain inspections of those greases. Not only does the process of the present invention improve the distribution of the dilithium salicylate, but it also improves the dropping point of the grease and the effectiveness of the dilithium salicylate as an antioxidant.

The static oxidation test for which data are given in Table II was conducted in the following manner. Five grams of the grease being tested were packed into an open ball bearing of known weight which was then hung in an oven maintained at 350. F. At periodic intervals the hearing was removed from the oven and weighed again to determine weight loss of the grease. The results were reported in terms of the number of hours for a 35% weight loss.

TABLE II.PROPERTIES OF LITHIUM HYDROXYSTEA- RATE-DILITHIUM AZELATE GREASES Base Grease grease F G H Composition, parts by weight:

Lithium IZ-hydroxystearate- 93.6 93. 6 93. 6 93. 6 Dilithium azelate 45. 9 45. 9 45. 9 45. 9 Mineral oil, 90 V.I., 550 SUS at 100 F 917. 830. 917. 0 917. 0 Dilithium salieylate, preformed powder (3%) 32. 7 Dilithium salicylate (3%), made in situ via methyl salieylate 30. 0 32. 7

ASTM penetration:

77 F. nun/10:60 strokes 291 305 304 300 77 F. mm./10:100,000 strokes u 335 ASTM dropping point, F 390 525 +500 374 Static oxidation test, 350 F.,

hours to 35% weight loss 80 280 295 175 To further illustrate the invention, the preparation of Grease F can be repeated, substituting the ethyl ester of 2-hydroxy, 4-methyl benzoic acid or the isopropyl ester of parahydroxy benzoic acid for methyl salicylate.

This invention is not to be limited to the specific examples given herein by way of illustration. Its scope is defined by the appended claims.

What is claimed is:

1. An improved method of incorporating an oil-insoluble alkali metal salt of a hydroxy benzoic acid in a min eral lubricating oil composition which comprises adding to said composition an aliphatic alcohol ester of said hydroxy benzoic acid, reacting said ester with an alkali metal base, and removing from said composition the Water and alcohol formed in the reaction.

2. Improvement as defined by claim 1 wherein said ester is an ester of a C to C aliphatic alcohol.

3. Improvement as defined by claim 1 wherein said ester is an ester of a C to C aliphatic alcohol.

4. Improvement as defined by claim 1 wherein said mineral lubricating oil composition is a grease and said reaction of ester and base is conducted during the neutralization step in the soap-making stage of manufacturing said grease.

5. Improvement as defined by claim 1 wherein said alkali metal salt is dilithium salicylate.

6. Improvement as defined by claim 1 wherein said ester comprises methyl salicylate.

References Cited UNITED STATES PATENTS 2,951,808 9/1960 Norton et a1. 25241 DANIEL E. WYMAN, Primary Examiner P. E. KONOPKA, Assistant Examiner US. Cl. X.R. 

